You cant just renew automatically

This is the second time now that i have asked that a contract be cancelled and it just gets upgraded. I received a print out from the Fourways Vodacom Store that my contract is eligible for upgrade 2018-03-10 so i have it in writing. I waited til 30 May 2018 to give one months cancellation notice. Spoke to Velicia, she said the contract runs one month in arrears and so last payment will be end of June 2018. I notice i am still paying for it so i wrote only to be told today no, the contract will only end 4 May 2019 - but how? Where did i sign for it?